Rendezvous with an asteroid Rubin, Alan E
Science (American Association for the Advancement of Science),
03/2024, Letnik:
383, Številka:
6689
Journal Article
Recenzirano
A planetary scientist recounts an audacious mission to retrieve mineral samples from space
Advances in high-throughput functional genomics have enabled researchers to measure many thousands of individual genetic variants in a single gene in parallel using techniques such as deep mutational ...scanning (Fowler and Fields,
2014
). The success of these approaches depends on the availability of assays that can measure a wide range of protein functions. In their recent work, Kudla and colleagues (McDonnell et al,
2024
) applied deep mutational scanning to the transcription factor
PAX6
, which has a key role in eye development, and described a new high-throughput functional assay that could be applied to almost any transcription factor. The authors successfully measured the effects of 95% of missense variants in PAX6 and show that their assay results are highly concordant with known clinical variants. Notably, they also undertook a wide-ranging survey of computational variant effect predictors and show that their experimental data outperformed cutting-edge algorithms.
AF Rubin discusses a new high-throughput functional assay for transcription factors applied for a deep mutational scanning study of the transcription factor PAX6 by Kudla and colleagues (McDonnell et al,
2023
) in this issue of
Molecular Systems Biology
.
CM chondrites exhibit a strong correlation between the degree of alteration and the extent of particle alignment (i.e., the strength of the petrofabric). It seems likely that the S1 shock stage of ...essentially every CM and the high matrix abundance (∼70vol.%) of these samples ensured that the shock waves that produced CM petrofabrics (by collapsing matrix pores and squeezing chondrules into pore spaces) were significantly attenuated and were too weak to damage olivine crystal lattices. Random collisions on the CM body produced petrofabrics and created fractures in the target rocks. Subsequent impact-mobilization of water caused hydrated phases to form preferentially in the more-fractured regions (those with the strongest petrofabrics); the less-deformed, less-fractured CM regions experienced lower degrees of aqueous alteration.
Many CV3 chondrites also have petrofabrics: roughly half are from the oxidized Bali-like subgroup (CV3OxB), roughly half are from the reduced subgroup (CV3R) and none is from the oxidized Allende-like subgroup (CV3OxA) (which is less altered than CV3OxB). Nearly all CVs with petrofabrics are S3–S4 and nearly all CVs that lack petrofabrics are S1. Oxidized CVs have much higher porosities (typically 20–28%) than reduced CVs (0.6–8%), facilitating more-extensive aqueous alteration. The CV3R chondrites formed from low-porosity material that inhibited oxidation during alteration. The oxidized CV subgroups formed from higher-porosity materials. The CV3OxB samples were shocked, became extensively fractured and developed petrofabrics; the CV3OxA samples were not shocked and never developed petrofabrics. When water was mobilized, both sets of porous CV chondrites became oxidized; the more-fractured CV3OxB subgroup was more severely altered.
Paris is the least aqueously altered CM chondrite identified to date, classified as subtype 2.7; however, literature data indicate that some regions of this apparently brecciated meteorite may be ...subtype 2.9. The suite of CAIs in Paris includes 19% spinel–pyroxene inclusions, 19% spinel inclusions, 8% spinel–pyroxene–olivine inclusions, 43% pyroxene inclusions, 8% pyroxene–olivine inclusions, and 3% hibonite‐bearing inclusions. Both simple and complex inclusions are present; some have nodular, banded, or distended structures. No melilite was identified in any of the inclusions in the present suite, but other recent studies have found a few rare occurrences of melilite in Paris CAIs. Because melilite is highly susceptible to aqueous alteration, it is likely that it was mostly destroyed during early‐stage parent‐body alteration. Two of the CAIs in this study are part of compound CAI–chondrule objects. Their presence suggests that there were transient heating events (probably associated with chondrule formation) in the nebula after chondrules and CAIs were admixed. Also present in Paris are a few amoeboid olivine inclusions (AOI) consisting of relatively coarse forsterite rims surrounding fine‐grained, porous zones containing diopside and anorthite. The interior regions of the AOIs may represent fine‐grained rimless CAIs that were incorporated into highly porous forsterite‐rich dustballs. These assemblages were heated by an energy pulse that collapsed and coarsened their rims, but failed to melt their interiors.
•Maskelynite is a diaplectic glass formed from plagioclase shocked to ∼20–30GPa.•The proportion of basalts with maskelynite depends on parent-body escape velocity.•Maskelynite: eucrites, 5%; lunar ...basaltic meteorites, 30%; shergottites, 93%.•Maskelynite occurs in 1% of Apollo basalts, which were never launched off the Moon.•Vesta ejecta experienced the lowest shock pressures, martian ejecta the highest.
Maskelynite is a diaplectic glass that forms from plagioclase at shock pressures of ∼20–30GPa, depending on the Ca concentration. The proportion of maskelynite-rich samples in a basaltic meteorite group correlates with the parent-body escape velocity and serves as a shock indicator of launching conditions. For eucrites (basalts widely presumed to be from Vesta; vesc=0.36kms−1), ∼5% of the samples are maskelynite rich. For the Moon (vesc=2.38kms−1), ∼30% of basaltic meteorites are maskelynite rich. For Mars (vesc=5.03kms−1), ∼93% of basaltic meteorites are maskelynite rich. In contrast, literature data show that maskelynite is rare (∼1%) among mare basalts and basaltic fragments in Apollo 11, 12, 15 and 17 soils (which were never ejected from the Moon). Angrites are unbrecciated basaltic meteorites that are maskelynite free; they were ejected at low-to-moderate shock pressures from an asteroid smaller than Vesta.
Because most impacts that eject materials from a large (⩾100km) parent body are barely energetic enough to do that, a collision that has little more than the threshold energy required to eject a sample from Vesta will not be able to eject identical samples from the Moon or Mars. There must have been relatively few impacts, if any, that launched eucrites off their parent body that also imparted shock pressures of ∼20–30GPa in the ejected rocks. More-energetic impacts were required to launch basalts off the Moon and Mars. On average, Vesta ejecta were subjected to lower shock pressures than lunar ejecta, and lunar ejecta were subjected to lower shock pressures than martian ejecta.
H and LL ordinary chondrites have low percentages of shock-stage S5 maskelynite-bearing samples (∼1% and ∼4%, respectively), probably reflecting shock processes experienced by these rocks on their parent asteroids. In contrast, L chondrites have a relatively high proportion of samples containing maskelynite (∼11%), most likely a result of catastrophic parent-body disruption 470Ma ago.
Iron‐meteorite groups that appear from published isotopic data to have been derived from melted carbonaceous‐chondrite‐like precursors (CC irons) (IIC, IID, IIF, IIIF, IVB) tend to have higher median ...refractory siderophile element (RSE) contents, higher median Ni contents, and higher median Ir/Ni and Ir/Au ratios than magmatic noncarbonaceous (NC) iron‐meteorite groups (IC, IIAB, IIIAB, IIIE, IVA). (Group IIG is also NC.) One potential source of RSEs in magmatic CC irons is the set of refractory metal nuggets from inherited CAIs. Magmatic CC‐iron groups tend to have longer cosmic‐ray exposure (CRE) ages than magmatic NC‐iron groups, indicating long residence times as small bodies in interplanetary space. The lower membership of CC‐iron groups is probably mainly due to the high oxidation state of their precursors. Such oxidation would have produced lesser amounts of free metal; parent body differentiation of such bodies would have produced smaller cores, resulting in fewer samples available to make CC‐iron meteorites in the first place. (Ungrouped magmatic irons, most of which can be considered groups with only one member, also tend to be carbonaceous.) It is possible that a subset of the chondrule‐poor dark inclusions in many carbonaceous chondrites represent unmelted materials related to the precursors of the CC irons. The Eagle Station pallasites (also CC‐related) are analogous to CC irons in being more oxidized, richer in Ni and RSEs, and fewer in number than main‐group pallasites (PMG). However, Eagle Station has a shorter CRE age than most PMG.
Chondrite groups (CV, CK, CR) with large average chondrule sizes have low proportions of RP plus C chondrules, high proportions of enveloping compound chondrules, high proportions of chondrules with ...(thick) igneous rims, and relatively low proportions of type-I chondrules containing sulfide. In contrast, chondrite groups (CM, CO, OC, R, EH, EL) with smaller average chondrule sizes have the opposite properties. Equilibrated CK chondrites have plagioclase with relatively low Na; equilibrated OC, R, EH and EL chondrites have more sodic plagioclase. Enveloping compound chondrules and chondrules with igneous rims formed during a remelting event after the primary chondrule was incorporated into a dustball. Repeated episodes of remelting after chondrules were surrounded by dust would tend to produce large chondrules. RP and C chondrules formed by complete melting of their precursor assemblages; remelting of RP and C chondrules surrounded by dust would tend to produce porphyritic chondrules as small dust particles mixed with the melt, providing nuclei for crystallizing phenocrysts. This process would tend to diminish the numbers of RP and C chondrules. Correlations among these chondrule physical properties suggest that chondrite groups with large chondrules were typically surrounded by thick dust-rich mantles that formed in locally dusty nebular environments. Chondrules that were surrounded by thick dust mantles tended to cool more slowly because heat could not quickly radiate away. Slow cooling led to enhanced migration of sulfide to chondrule surfaces and more extensive sulfide evaporation. These chondrules also lost Na; the plagioclase that formed from equilibrated CK chondrites was thus depleted in Na.
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